1. Field of the Invention
This invention relates to a mutual inductance current transducer, method of making same, and an AC electric energy meter incorporating same and, more particularly, to a mutual inductance current sensing-transducer having features of reduced size, affording significant simplification and reduced cost of production, greatly reduced capacitive shielding requirements and elimination of magnetic shielding requirements in an air core embodiment and, in an electric energy meter incorporating same, contributing to simplification of the meter assembly and to reduction of the size requirements, weight and cost of the meter.
The improved mutual inductance current sensing transducer of the present invention and an electric energy meter incorporating same relate to and represent an improvement over the subject matter of U.S. Pat. No. 4,413,230--Miller, assigned to Westinghouse Electric Corp., the assignee of the present invention, and the disclosure of which is incorporated herein. The '230 patent sets forth, in its "Description of the Prior Art," a helpful discussion of prior types of AC electric energy measuring circuitry which is useful as well to an understanding of the present invention.
One of the preferred embodiments of the present invention has particular application to so-called 200 amperes, single-phase service as typically is supplied to residential users, although in another of the preferred embodiments it may be employed for 20 amperes multi-phase service.
At a typical utility customer location, 60 cycle (Hertz), single-phase AC electric power is delivered at a substantially constant line voltage of 240 volts, thus defining the voltage component of the electrical energy quantity to be measured. As is conventional, the electric power is supplied on three conductors, the center conductor being grounded and the two hot conductors each being at 120 volts in 180.degree. out-of-phase relationship, so as to provide 240 volts total therebetween. Accordingly, two hot conductors carrying 120 volts AC power, relative to the third, ground conductor, pass through the meter Voltage and current transducers sense the magnitude of the voltage and current components of the power supplied through the meter to the loads at the customer location, and produce corresponding signals representative of those sensed components which are processed for determining the power consumed. Whereas the voltage component of the power remains substantially constant, the currents through the two conductors, which together define the current component of the electrical energy quantity to be measured, vary considerably in response to load changes. In general, the current component has levels extending throughout a range of from one-half ampere to 200 amperes, at a minimum--i.e., a current variation ratio of approximately four hundred to one. This rather substantial range, or current level variation ratio, requires that the current sensor have a substantially linear response for achieving accurate measurement of power consumption by the meter, for billing purposes.
Accordingly, standard potential transformer arrangements can provide practical voltage sensing transducers. However, current transformers capable of providing a linear response to current flows which are subject to such-wide variations, in the order of four hundred to one, and producing low voltage level signal outputs are often of substantial size and cost. As is known, in accurate current transformer transducers, the ampere turns of the primary and of the secondary must be equal, and since current levels can produce 400 ampere-turns in the primary, the secondary winding sizes become substantial in order to produce linear low voltage level signal outputs. As a result, conventional current transformers which can satisfy these requirements are bulky and relatively costly.
As disclosed in the referenced '230 patent, it has long been recognized to be desirable to provide current sensing transducers for electronic AC energy measuring circuits which are highly reliable and accurate, which are adapted for standard connection to the conductors supplying the electric energy to be measured, such as the service entrance conductors of a residential electric power user's location, and which are compact in size and capable of mass production by economical manufacturing techniques. Such transducers must be operable to produce low level analog voltage signal outputs which are accurately responsive to the large variations of load currents to be sensed.
The '230 patent discloses various embodiments of electric watt-hour meter circuits designed to satisfy these criteria. In general, each of those embodiments comprises a mutual conductance current sensing transducer having secondary winding means inductively coupled to primary winding means, comprising a current conductor, which carries a current component of the electrical energy to be measured. Each of the transducer embodiments disclosed therein is responsive to the above-noted wide ratio of current variations, the secondary winding producing analog voltage output signals which are proportional to the time derivative of the current, and are suitable for processing with voltage-responsive output signals of a voltage transducer for developing AC electric energy measurement output signals representative of the power actually consumed.
The flow of current in the primary winding produces a magnetic flux field which is coupled to the secondary winding of the sensor, the latter being in sufficiently close inductive relationship to the primary winding to develop an induced voltage, e.sub.i =M di/dt, where M is the mutual inductance between the primary and secondary windings and di/dt is the time derivative of the primary current. In accordance with the above equation, the secondary winding output signal e.sub.i is representation of the time derivative of the primary current when the primary and secondary windings are mutually coupled, with or without a magnetic core. The induced, analog voltage output signal e.sub.i of the secondary winding is provided as a current responsive, analog input signal to an electronic AC energy measuring circuit. The meter also comprises a voltage sensor which is coupled to the current conductor and supplies a voltage responsive analog input signal e.sub.v which likewise is applied to the measuring circuit. The measuring circuit processes the signals e.sub.i and e.sub.v to produce a signal representative of the alternating current energy consumption. More specifically, the circuit derives the time integral of the product of the voltage and current components of the electrical energy quantity over a predetermined time interval, thereby to provide energy measurement in watt-hours. A significant factor of the measurement circuit is that it has a very high input impedance and thus effectively no energy needs to be coupled from the current flow by the current sensing transducers, this factor contributing significantly to the compactness and efficiency of the current sensing transducers of the '230 patent.
One of the preferred embodiments of the mutual inductance current sensing transducers of the '230 patent comprises a pair of toroidal secondary windings which are inductively coupled through corresponding air spaces to respective heavy gauge current conductors, each of the latter effectively forming a single turn primary winding. The air space coupling refers to the fact that the core of the toroidal secondary winding is of a non-magnetically permeable material, such as plastic, and offers the benefit of affording an absolutely linear response characteristic of the transducer. The current conductors are connected to conventional blade terminals and disposed within a meter frame, permitting conventional attachment of the meter to mating sockets of existing metering locations. A voltage sensing transducer is also mounted within the meter frame and connected between the current conductors to produce an analog voltage output signal e.sub.v. The output of the toroidal secondary windings are summed in series to produce a current responsive analog voltage signal output e.sub.i, representative of the sums of the separately sensed line currents in the conductors. The voltage sensor and current sensor outputs then are supplied to a processing circuit, as above described. The air core toroidal secondary windings of the '230 patent offer the benefits of a very wide dynamic range and absolute linear response characteristics. Further, because of the high input impedance of the processing circuit, and the effective absence of any need to couple energy from the conductors through the current sensors, the toroidal coils, or secondary windings, may be formed of very small gauge wire for producing the requisite analog voltage signal output levels representative of the range of current variations to be sensed.
While such current sensing transducers and associated meters incorporating same, as disclosed in the '230 patent, have achieved wide-spread acceptance in actual commercial use, there have been on-going, continuing investigations into improvements, for example to achieve size and cost reductions of the toroidal sensors and, ultimately, reduced costs and simplification of the production of meters incorporating same. The development efforts particularly addressed the desire to maintain the beneficial aspects of the toroidal air core secondary windings, in view of their beneficial characteristic noted above.
In practice, it was recognized that by maintaining precise control in the winding of the toroidal sensing coil so as to achieve effectively perfect symmetry, inherent cancellation of magnetic interference due to external, or extraneous, magnetic fields coupled to the toroidal coil could be achieved. Further, in the single phase, 200 amps, three-wire metering applications as taught by the '230 patent, the opposite directions of current flow through the two current conductors within the meter afforded mutual cancellation of magnetic interference caused by extraneous fields coupled to the toroidal coils of the respective current sensors, due to the series-summing connection of their outputs. In ideal cases and in certain practical applications, the need for magnetic shielding thus could be avoided. In many practical applications, however, magnetic shielding was required.
The '230 patent further teaches the provision of capacitive shielding of the toroidal coils of the current sensors. A thin layer of highly conductive material, such as copper, is formed as a cylindrical sleeve coaxially about a corresponding, cylindrical insulating sleeve positioned on the conductor/primary winding and the toroidal coil is coaxially positioned on and mounted to the sleeve at a position intermediate its ends. Further, a cup-like metallic shield is formed about the exterior of the toroidal coil and electrically connected to the sleeve and grounded. Where magnetic shielding as well is required, the cup-like shield is made of a suitable magnetically permeable material. In practice, due to the inability to eliminate sources of magnetic interference at the metering installations, routine practice dictated that the toroidal secondary windings, or coils, be precision wound to maintain near-perfect symmetry to achieve the inherent cancellation effects above noted, and that the cup-like shield provide both magnetic and capacitive shielding.
Various efforts were undertaken to define simplified ways to reduce or effectively cancel the adverse effects of the extraneous magnetic fields on the toroidal coils and also for reducing the costs of production, but without success. For example, techniques were pursued for achieving precise winding of the toroidal coils, in a single layer of turns so as to maintain essentially perfect symmetry, the toroidal coils being otherwise of the conventional type employed in meters for residential 200 amperes service as above-noted and as disclosed in the '230 patent. The concept was directed to optimizing the inherent cancellation of extraneous or external magnetic fields by each toroidal coil, due to the essentially perfect symmetry. This, however, required extraordinary precision controls not attainable in normal production operations, due to usual machine tolerances and the like. Other efforts were directed toward developing a magnetic shield structure, such as by providing external iron shields of low cost which would be effective against extraneous, interfering magnetic fields. These efforts, however, required excessively large shielding structures incompatible with the space limitations within the meter, and were soon recognized to be unattractive and unacceptable. Moreover, these and other efforts directed to providing soft magnetic shields of various types had the adverse effect of causing a slope, or tilt, to the current sensor response curve, rendering it incapable of providing the desired linear response over the required range, or ratio, of the variations in current levels to be sensed, i.e., the minimum 400 to 1 ratio above-mentioned.
Accordingly, there has been a continuing need for achieving a toroidal coil secondary winding, for use in a differential current sensor, which is of reduced manufacturing costs, but which provides the requisite linear response and wide dynamic range characteristics, and produces output voltages of the required levels so as to be compatible with existing processing circuits--but which is essentially, inherently, unaffected by extraneous magnetic and electrostatic fields thereby to minimize or eliminate the need for magnetic and electrostatic shielding structures. Necessarily, it was recognized that a toroidal coil usable in a differential current sensor and meeting these optimistic goals would contribute significantly to a reduction in the internal space requirements therefor within an electric meter and, correspondingly, in the complexity of assembling such meters and in their ultimate cost of production.